1. Field of the Invention
This invention generally relates to a photosensor for converting light information into an electrical signal, and, in particular, to an amorphous silicon photosensor having a multi-layer, sandwich type structure in which two or more amorphous silicon layers are sandwiched between a pair of electrodes. More specifically, the present invention relates to a photosensor suitable for use as an image sensor in the likes of a facsimile machine.
2. Description of the Prior Art
Photosensors are well known in the art and they may be classified into those of the CdS-CdSe family and those of the Se-Te-As family. However, in the case of the CdS-CdSe family, the light response speed is relatively slow; whereas, in the case of photosensors of the Se-Te-As family, these photosensors exhibit the drawback of crystallization at low temperature. And, it is still difficult to obtain an excellent photosensor high in density and fast in operation. In addition, there still remains pollution problems. In order to cope with this situation, there has been developed an amorphous silicon photosensor free of pollution problems, high in light absorption coefficient as well as in dark resistivity, excellent in photoconductivity, and easy in providing a large surface area as well as forming a thin film.
In accordance with a typical prior art amorphous silicon photosensor, a set of electrodes is formed on one surface of a substrate and an amorphous silicon layer is formed as extending between the electrodes. This type of prior art amorphous silicon photosensor is called a coplanar type amorphous silicon photosensor and a relatively large number of sample products have been experimentally manufactured because of its ease in manufacture. However, due to its structural constraints, the light response speed is relatively slow and there is difficulty in application to high-speed facsimilies. This is because, in the coplanar type amorphous silicon photosensor, the distance between a set of electrodes is required to be relatively large, i.e., in the order of 10 microns, so that it takes time for carriers, such as electrons, to move between the set of electrodes across the gap therebetween. In addition, since the set of electrodes is formed on the same plane, there is a limit in density as well.
In order to cope with this situation, there has also been developed a sandwich type amorphous silicon photosensor in which an amorphous silicon layer is sandwiched between a transparent electrode and another electrode. It is expected that this sandwich type amorphous silicon photosensor will provide a desired high-density and high-speed photosensor. However, in this sandwich type photosensor, since charge is injected from electrodes while a bias voltage is being applied between the electrodes, the dark current tends to be larger, so that there is a drawback of incapability of obtaining a large light and dark ratio (S/N ratio), which is a ratio between current with light and current without light. Amorphous silicon has a relatively large light absorption coefficient and thus it may be made in the form of a thin film (typically, ranging between 5,000 angstroms and 1 micron), which can contribute to provide a high-speed operation. However, when made in the form of a thin film, there is a possibility of shorting between the electrodes through pin holes in the thin film, thereby preventing another difficulty in providing high yields of photosensors which are uniform in characteristics.
In order to reduce the dark current of a sandwich type photosensor, there has also been proposed a MIS type amorphous silicon photosensor having a blocking structure for blocking the injection of charge from the electrodes by additionally providing a thin insulating film between amorphous silicon and the electrodes. An insulating film of silicon oxide, silicon nitride or metal nitride has been proposed for this purpose. For example, according to teachings of the Japanse Patent Laid-open Pub. No. 57-106179, it is proposed to subject the surface of amorphous silicon to anodic plasma oxidation to form a thin insulating film or to cause glow discharge in silane gas containing oxygen to form a silicon oxide film on the amorphous silicon. According to the teachings of this patent application, the preferred thickness of the resulting insulating film is in the order of 20-40 angstroms, and, thus, it is rather difficult to form such a film across the entire surface of high-density photoelectric elements uniformly in composition as well as in thickness. Because, if the thickness of a thin film is less than 100 angstroms, the resulting thin film tends to be patchy so that there is a difficulty in obtaining a uniform characteristic.
On the other hand, Japanese Patent Laid-open Pub. No. 56-26478 teaches the use of silicon nitride (light-transmitting, current-passing, insulating or semi-insulating), which is lower in resistivity than silicon oxide, as the above-described insulating thin film. In the case of silicon oxide, since it has a relatively large energy bandgap, it tends to resist the passage of current therethrough even if it is made in the form of a thin film, so that it is difficult for carriers to move across the thin film of silicon oxide during light irradiation. As a result, there is a drawback of difficulty in obtaining a high light and dark ratio (S/N ratio) in the case where use is made of silicon oxide. Under the circumstances, this patent application proposes to use silicon nitride, which has a smaller energy bandgap as compared with silicon oxide and which is lower in resistivity than silicon oxide. However, even according to the teachings of this patent application, the preferred thickness of a resulting silicon nitride thin film is in the order of 50-100 angstroms, so that there still remains difficulty in obtaining a photosensor uniform in characteristic.
In the case of fabricating a high-density photosensor, typically, individual electrodes are formed and then an insulating film is formed. In this case, the individual electrodes are formed by etching with a photolithographic technique prior to formation of the insulating film. However, since it is difficult to form a thin insulating film on the shoulder portion along a side edge defined by such etching, it is even more difficult to fabricate a photosensor having a uniform characteristic.
Furthermore, the Japanese Patent Laid-open Pub. No. 56-14268 discloses a photoconductive semiconductor device which includes a multi-layer structure of amorphous silicon, in which at least one layer of amorphous silicon includes oxygen and impurities for controlling charge density. However, if oxygen is present and the optical bandgap (Eg-opt) is widened, a photoconductive characteristic is shown; however, if the optical bandgap is further widened, no photoconductive characteristic is shown. In order to use a highly resistive thin film so as to block introduction of charge, it is necessary to used amorphous silicon having an optical bandgap of 2.0 eV; in this case, however, difficulty is presented for the carrier which has been generated upon irradiation of light to pass, so that a photosensor having a large light and dark ratio (S/N ratio) cannot be obtained. That is, under the current status, there has not been obtained amorphous silicon having a high resistance (i.e., large optical bandgap) and photoconductivity particularly suitable for use in a sandwich type amorphous silicon photosensor.